Please refer to FIG. 1 which is a schematic diagram of a well-known magnetron circuit. As shown in FIG. 1, the magnetron is a vacuum tube for generating microwave. Under its normal working conditions, when its cathode temperature is over 2100° K (absolute temperature), a negative high voltage of several thousand volts is applied between a cathode and a anode of the magnetron. However, different magnetrons have various values of the working voltages. The characteristics of voltage verse current relationship substantially are the similar. As illustrated in FIG. 2, when the voltage between the cathode and the anode reaches to a working voltage, the magnetron emits a microwave. After the voltage between the cathode and the anode is clamped or held to the working voltage, the characteristic of the magnetron is used to be deemed as a voltage stabilizing tube.
Please refer to FIG. 3 which is a circuit schematic diagram of a well-known forward-flyback converter. As illustrated in FIG. 3, the working principle of the well-known forward-flyback converter 100 is as follows: A driving signal of a main switch 101 and an auxiliary switch 102 is a complementary signal. A fifth capacitor 103 is employed in the converter to clampe and control the primary winding voltage of a transformer 104 and to magnetically reset the transformer 104.
Please refer to FIG. 4 which is a circuit waveform schematic diagram of the well-known forward-flyback converter. In FIG. 4, VGS1 is a driving signal of the main switch 101, VGS2 is a driving signal of auxiliary switch 102, I1 represents a conductive current of the main switch 101, and I2 represents a conductive current of auxiliary switch 102. The advantages of the well-known forward-flyback converter are described as follows: (1) The main switch 101 and the auxiliary switch 102 are turned on by zero-voltage-switch (ZVS), (2) The rectifying diode of the secondary winding is cut off by zero-current-switch (ZCS), there are no reverse recovery problem. The drawbacks of the well-known-forward-flyback converter are described as follows: (1) Because the capacitance of the first filtering capacitor 105 is small, in order to reduce a current ripple of a first filtering inductor 106, the inductance of the first filtering inductor 106 must be enlarged. (2) Because the direct current bias value of the magnetic flux in a high voltage transformer is high, in order to prevent the transformer from operation at saturation state, the air gap in the core of the transformer should increase, therefore, the loss of the transformer increase.
For facilitating understanding the problem of the direct current bias value of the transformer, it is explained as follows: FIG. 5 is a transformer equivalent circuit of the well-known forward-flyback converter. Numeral 107 is an excited inductor of the primary winding of the transformer 104. Because a direct current portion of a current can not flow through a seventh and sixth capacitors 108 and 109, no direct current portion of a current flow through the transformer 104. The mean-square-value current flowing through the excited inductor 106 is equal to Iin, and an excited current peak value is Im. Assume that the power factor of the power supply is 1, then iin, Pin, Im, Im max are calculated in the following equations (1)-(4).iin=Im sin ωt  (1)Pin=VinIin=Pout/η  (2)Im=√{square root over (2)}Iin=√{square root over (2)}Pout/Vinη  (3)Im max=√{square root over (2)}Iin max=√{square root over (2)}Pout max/Vin minη  (4)wherein,                iin represents an input current.        Pin represents an average input power        Vin represents a mean-square-value of an input voltage        Iin represents a mean-square-value of an input current        Pout represents a average output power        η represents efficiency of a transformer        
Moreover, a direct current bias peak value of a magnetic potential in the transformer core is illustrated in the following equation (5).Udc max=NIm max  (5)wherein, N represents a coil number of a primary winding
However, the direct current bias value of magnetic potential is very large under conditions of full load and low input voltage. Therefore, the utilization rate of the magnetic core in the transformer is low. Thus, a large air gap must exist in the magnetic core of the transformer. Hence, the loss of the transformer is enlarged.
Therefore, in order to solve the above problem and the drawbacks of prior art, this invention provides a high frequency heating device.